Reply to "Tires for OEM 15 inch wheels"

quote:
Originally posted by tajon:

Why is this George?


A front tire 25 inches in diameter fills the front fender arch quite fully, whereas a front tire smaller than 24 inches in diameter begins to look proportionally too small for the fender arch. Therefore front tire diameter should remain within the range of 23.9” to 25.0”. A rear tire 27 inches in diameter fills the rear fender arch quite fully, whereas a rear tire smaller than 26 inches in diameter begins to look proportionally too small for the rear fender arch. Therefore rear tire diameter should remain within the range of 25.9” to 27.0”. There is an aesthetic condition that must be considered when selecting tires. The front tires and rears tires should “fill” the front and rear fender arches equally. If a 25 inch diameter front tire is combined with a 26 inch diameter rear tire, there would be a situation where the front tire fills the front fender arch fully, while the rear tire would have a rather large clearance between it and the rear fender arch. There would be an aesthetic imbalance in the appearance of the car. As a result of this, the front and rear tires must go up or down in diameter together, or to put this another way the rear tires should be about 2 inches greater in diameter than the front tires (+/- 0.25 inch) in order for your Pantera to look its best. Therefore the diameter of the rear tire should be no less than 1.7” and no more than 2.3” larger than the front tire.

This also impacts the chassis ride height, i.e. the mechanical performance of the chassis. As the difference between front tire OD and rear tire OD decreases (less than 1.7") or increases (greater than 2.3") it "excessively" alters the chassis intended level ride height. This can be compensated for if the car has shocks with adjustable spring pre-load, but if the car has OEM shocks or red body Konis it can't.


Quickly, to set up a Pantera chassis with shocks having adjustable spring pre-load:

(1) set the rear ride height for level lower control arms (lowest setting, closest to OEM)

(2) set the front ride height to level the chassis. The chassis between the axles should be level with the ground, parallel to the ground, equal distance along its length to the ground. If you place a 2 foot bubble level on the door sills, they should be level. Leveling the chassis is important to give the chassis the "balance" that the designer intended it to have.

Changes to the front ride height will alter the bump-steer characteristics of the steering geometry, and the angles of the pivot axis of the upper and lower control arms. Changes to the rear ride height will alter the angles at which the half-shafts (and therefore u-joints) must operate at, plus the angles of the pivot axis of the upper and lower control arms. Combinations with greater front to rear difference in tire diameter make it necessary to raise the front of the car to level the chassis, this increases the gap between the front tire and front wheel arch. Combinations with less front to rear difference in tire diameter make it necessary to lower the front of the car to level the chassis; this reduces the gap between the front tire and the front wheel arch. Some guys do this intentionally in order to lower the CG of the car. Unfortunately this reduces ground clearance, and makes it hard to get a jack under the car.

The interaction of these parameters doesn't leave much room for customization, not like a good ol' American muscle car. But there's a lot of performance built into the Pantera chassis that those muscle cars will never have. If the front and rear tires are of the proper diameter, and their diameters are about 2" apart front to rear, and once the chassis is level, then control arm geometry, bump steer, front caster and appearance should all be "in the ball park". By appearance being in the ball park I mean the gap between the outside radius of the tires and the fender arch radius should be proportionally equal front tires to front fenders and back tires to back fenders. The tires should also be centered very well in the wheel arches. The factory set front caster should be at least 2-1/2 degrees, which is actually inadequate, it was originally intended to be 6 degrees, and should be at least 4 degrees. This causes the tires to lean into the turns, and gives the tires a better grip in corners. Ford intentionally dumbed-down the caster to 2-1/2 degrees in order to make the handling less responsive to driver input, i.e. that was how they "slowed" the cars handling. Ford also screwed-up the chassis set-up with the spacers they placed between the shocks and springs. and then there's the smaller rear anti-sway bar too.

The gearing, odometer, speedometer and rear wheel arch were all set-up for about 26.9" diameter rear tire. If the rear tires should be about 2" larger OD than the front tires, that means the front tires should be about 24.9" diameter.

Hope that gives you a feel for the interaction between all the components, and why we shouldn't stray to far from the originally intended settings.
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